A secondary battery is a device that stores electrical energy in chemical form and converts the stored chemical energy into electrical energy to generate electricity when needed. The secondary battery is also referred to as a rechargeable battery because it can be recharged repeatedly. A common secondary battery includes a lead accumulator, a NiCd battery, a NiMH accumulator, a Li-ion battery, and a Li-ion polymer battery. When compared to a disposable primary battery, not only is the secondary battery more economically efficient, it is also more environmentally friendly.
A secondary battery is currently used in applications requiring low electric power, for example, equipment to start a vehicle, a mobile device, a tool, and an uninterruptible power supply. Recently, as the development of wireless communication technology has been leading to the popularization of mobile devices and even to the mobilization of many types of existing devices, the demand for a secondary battery has been dramatically increasing. A secondary battery is also used in an environmentally friendly next-generation vehicle such as a hybrid vehicle or an electric vehicle to reduce the cost and weight and to increase the service life of the vehicle.
Generally, most secondary batteries have a cylindrical, prismatic, or pouch shape. This is associated with a fabrication process of a secondary battery that mounts an electrode assembly composed of a negative electrode, a positive electrode, and a separator in a cylindrical or prismatic metal can or a pouch-shaped case of an aluminum laminate sheet, and injects an electrolyte into the electrode assembly. Because a predetermined mounting space for the secondary battery is necessary in this process, the cylindrical, prismatic or pouch shape of the secondary battery is a limitation in developing various shapes of mobile devices. Accordingly, there is a need for a new type of secondary battery that is easily adaptable in shape.
In keeping with the need, suggestions have been made to develop a cable type secondary battery having a very high ratio of length to cross-sectional diameter. A cable type secondary battery may have a capacity reduction and deterioration in cycle life characteristics when a separation phenomenon of an electrode active material layer occurs due to stress caused by an external force in the event of deformation or rapid volume expansion of the electrode active material layer during charge/discharge.
To solve the problem, the binder content in the electrode active material layer increases and flexibility against bending or distortion will be provided. However, an increase in binder content in the electrode active material layer increases the electrode resistance, causing battery performance degradation. Further, if an external force strong enough to completely fold an electrode is applied, even though the binder content is increased, the separation of the electrode active material layer cannot be prevented, so this method does not present a proper solution.